The present invention relates to optical waveguides for use in optical components in the fields of optical communication and optical signal processing, and in particular to an optical waveguide Y junction for splitting input light received into an input waveguide path formed in a substrate into two parts.
Conventionally, junctions such as Y junctions have been used as splitters in optical integrated circuits (optical ICs) for use in the field of optical communication or the like. Such an optical waveguide junction typically comprises a substrate consisting of quartz glass or silicon wafer, and a core and upper clad layer which are formed over the substrate via an optional buffer layer formed over the substrate.
The use of a Y junction is not limited to simple splitting of light, but is also found in permitting communication between a light receiving element such as a photodiode placed at the terminal end of one of the output waveguide paths, and a light emitting element such as a laser diode placed at the terminal end of the other output waveguide path. In such a case, if the junction is designed such that the power of the input light, for instance, having the power of 10, is split into two equal parts or 5 for each output waveguide path, only half the power of the light emitting element reaches the light receiving element. Therefore, it has been necessary to compensate for such a loss of power by using a relatively sensitive light receiving element or a relatively powerful light emitting element.
To mitigate such a problem of the prior art, it has been proposed, for instance in Japanese patent laid open publications No. 8-122547 and No. 9-33740, to change the split ratio, for instance from 5/5 to 6/4 so that more of the light from the input waveguide path may be conducted to the photodiode or the light receiving element, and the power loss may be minimized.
According to the structure disclosed in Japanese patent laid open publication No. 8-122547, the splitting portion spreads out, and one of the output waveguide paths extends along the spread out profile while the other output waveguide path extends along a curved path. In other words, the plan view shapes of the two output waveguide paths are differentiated from each other so that an uneven split ratio may be achieved.
According to the structure disclosed in Japanese patent laid open publication No. 9-33740, the center of the two output waveguide paths is offset with respect to the input light waveguide path. Such a device may be called as a Y junction type optical tap.
According to the structure disclosed in Japanese patent laid open publication No. 8-122547, the downstream end of the input light waveguide path spreads out before it changes into the output waveguide paths, and the spread out part of the waveguide paths changes the light into a multi-mode light. The longer the spread out portion is, the greater the proportion of the undesirable higher waveguide mode components becomes. If the incident point of the input light deviates from the center of the input waveguide path due to an unavoidable error in the connection of optical fibers and the mounting of the junction, such a deviation has a strong influence on the split ratio.
According to the structure disclosed in Japanese patent laid open publication No. 9-33740, a large split ratio can be achieved, but the presence of a stepped portion in the optical waveguide path causes a significant loss. When a gradually spreading out portion is provided instead of a stepped portion, the problems such as the generation of higher waveguide modes as well as undesirable changes in the split ratio due the offsetting of the incident light arises.
According to these structures, a small offset in the order of xc2x12 xcexcm may well cause a deviation of xc2x110% or more in the split ratio.
It is conceivable to use optical fibers to split input light instead using a waveguide junction. In such a case, a pair of split optical fibers are required to be bonded to the input optical fiber, but a stable bonding is difficult to accomplish if the split ratio is not even as in the case of a 6/4 split ratio. A flawed bonding results in an unacceptably large loss, and a failure to achieve a desired split ratio.
In view of such problems of the prior art, a primary object of the present invention is to provide an optical waveguide Y junction which can split an input light at a desired split ratio by using a simple structure
A second object of the present invention is to provide an optical waveguide Y junction which can split an input light at a minimum loss.
A third object of the present invention is to provide an optical waveguide Y junction which can be fabricated relatively easily and at a high precision.
According to the present invention, such an object can be accomplished by providing an optical waveguide Y junction including an input waveguide path, a split portion for splitting a light input from the input waveguide path into two parts, and a pair of output waveguide paths for guiding the split parts of the input light, all formed in a substrate, characterized in that: the split portion has an outer profile which is symmetric with respect to a central axial line of the input waveguide path and identical to an outer profile of an optical waveguide Y junction for an even split ratio, and an inner profile which is asymmetric with respect to the central axial line so that light introduced from the input waveguide path may be split into the two output waveguide paths at an uneven split ratio. Preferably, the inner profile is defined by removing from a side of the split portion facing the output waveguide paths a wedge-shaped section having an oblique first side and a second side extending in parallel with the central axial line, on either side of the central axial line.
Thus, simply by changing the inner profile of the split portion, a desired uneven split ratio can be achieved without providing a spread out split portion or requiring any abrupt change in the shape of the waveguide path. This contributes to the reduction in the loss of the signal light. In particular, by selecting a wedge-shaped inner profile, the design and fabrication of the junction can be simplified. Also, a desired split ratio can be readily achieved by suitably changing the dimensions of the wedge-shaped section.
According to a certain aspect of the present invention, the optical waveguide Y junction comprises an input waveguide path; a split portion having one end smoothly connected to an end of the input waveguide path, and a pair of bifurcated ends; and a pair of output waveguide paths each having an end smoothly connected to a corresponding one of the bifurcated ends of the split portion; the split portion defining a modified asymmetric Y-shaped waveguide path which is defined by modifying an original symmetric Y-shaped waveguide path having a substantially constant width by removing an asymmetric wedge-shaped section from a side of the split portion interposed between the output waveguide paths.